Chris Meek

Seasonal variations of the semi-diurnal and diurnal tides in the MLT: multi-year MF radar observations from 2 to 70°N, and the GSWM tidal model

Abstract Continuous observations of the wind field have been made by six Medium Frequency Radars (MFRs), located between the equator and high northern latitudes: Christmas Islands (2°N), Hawaii (22°N), Urbana (40°N), London (43°N), Saskatoon (52°N) and Tromso (70°N). Data have been sought for the time interval 1990–1997, and typically 5 years of data have become available from each station, to demonstrate the level of annual consistency and variability. Common harmonic analysis is applied so that the monthly amplitudes and phases of the semi-diurnal (SD) and diurnal (D) wind oscillations are available in the height range of (typically) 75–95xa0km in the upper Middle Atmosphere. Comparisons are made with tides from the Global Scale Wave Model (GSWM), which are available for 3-month seasons. The emphasis is upon the monthly climatologies at each location based upon comparisons of profiles, and also latitudinal plots of amplitudes and phases at particular heights. For the diurnal tide, the agreement between observations and model is now quite excellent with modelled values frequently lying within the range of yearly values. Both observations and model demonstrate strong seasonal changes. This result is a striking improvement over the comparisons of 1989 (JATP, Special issue). In particular, the phases and phase-gradients for the non-winter months at mid- to high-latitudes are now in excellent agreement. Some of the low latitude discrepancies are attributed to the existence of non-migrating tidal components associated with tropospheric latent heat release. For the semi-diurnal tide, the observed strong transitions between clear solstitial states are less well captured by the model. There is little evidence for improvement over the promising comparisons of 1989. In particular, the late-summer/autumnal tidal maximum of mid-latitudes is observed to be larger, and with strong monthly variability. Also the summer modelled tide has unobserved short (20xa0km) wavelengths at high latitudes, and much smaller amplitudes than observed at all extratropical locations. Possible improvements for the GSWM’s simulations of the SD tide are discussed, which involve migrating tidal modes due to tropospheric latent heating.

Variations of the gravity wave characteristics with height, season and latitude revealed by comparative observations

Abstract This paper reviews some recent observations of gravity wave characteristics in the middle atmosphere, revealed by co-ordinated observations with the MU radar in Shigaraki (35°N, 136°E) and nearby rocketsonde experiments at Uchinoura (31°N, 131°E). We further summarize the results of comparative studies on the latitudinal variations of the gravity wave activity, which were detected by additionally employing data obtained with MF radars at Adelaide (35°S, 139°E) and Saskatoon (52, 107W) and lidar observations at Haute Provence (44, 6E). The seasonal variation of gravity wave activity detected with the MU radar in the lower stratosphere showed a clear annual variation with a maximum in winter, and coincided with that for the jet-stream intensity, indicating a close relation between the excitation of gravity waves and jet-stream activity at middle latitudes. The long-period (2–21 h) gravity waves seemed to be excited near the ground, presumably due to the interaction of flow with topography, and the short-period (5 min 2 h) components had the largest kinetic energy around the peak of jet-stream. We found an increase with height in the vertical scales of dominant gravity waves, which can be explained in terms of a saturation of upward propagating gravity waves. The values of the horizontal wind velocity variance generally increased in the stratosphere and lower mesosphere, but they became fairly constant above about 65 km due to the wave saturation, resulting in the active production of turbulent layers. Although the gravity wave energy showed an annual variation in the lower atmosphere, it exhibited a semiannual variation in the mesosphere, with a large peak in summer and a minor enhancement in winter. Lidar observations reasonably interpolated the seasonal variations in the intermediate height regions. The gravity wave energy in the mesosphere, with periods less than about 2 h, was consistently larger in summer than in winter at all the stations, i.e. at 35N, 44N,52 N and 35 S. However, the values were generally larger at 35 N than at 52 N. which was found from a comparison of l-yr observations at Shigaraki and Saskatoon. Furthermore, a comparison between Shigaraki and Adelaide, located at the conjugate points relative to the equator, revealed that the gravity-wave energy in the mesosphere was found to be fairly similar, when we compared the values in summer/winter in each hemisphere.

Global study of northern hemisphere quasi-2-day wave events in recent summers near 90 km altitude

Abstract We attempt to find the northern hemisphere zonal wavenumber for a striking quasi-2-day wave “event” or “burst” observed near 90 km altitude in the summer of 1992. A unique set of data on the upper atmosphere from nine radar sites is analysed (spacings ∼400– ∼ 12,000 km), and compared with expectations from models. The 2-day wave phase comparison, which finds zonal wavenumber m = 4, is conclusive. Determination of n, which defines the meridional wave amplitude structure, is not attempted, as the sites here have only a small latitude spread (21°N to 55°N). Also the amplitude seems to be unstable showing some sort of modulation which is not simultaneous at all sites. Finally, the radars have not been “calibrated” against each other in terms of wind speed. This calibration would have to be done before small differences in wave amplitude could be believed. A similar event in 1991 for which fewer sites are available is also discussed. Here the choice between m = 3 and 4 is not as clear.

Climatology of the semidiurnal tide at 52–56°N from ground-based radar wind measurements 1985–1995

Abstract Long-term wind measurements carried out at 6 northern midlatitude sites (Saskatoon, Sheffield, Juliusruh, Collm, Obninsk, Kazan) are investigated to establish a climatology of the semidiurnal tide in the mesopause region for the narrow latitudinal range between 52°N and 56°N. Comparison of zonal and meridional components shows that in general the horizontal components are circularly polarized. Intercomparison of amplitudes and phases generally shows good agreement between the results from the different measuring systems. The results are compared with an empirical model of the semidiurnal tide. The longitudinal variation of the semidiurnal tide is small in summer, but the tidal amplitudes in winter are larger at Saskatoon and Kazan, compared with the results from the other sites. The possible influence of wave–tidal interaction in the stratosphere on the interannual variability of this difference is discussed.

The quasi 16‐day oscillations in the mesosphere and lower thermosphere at Saskatoon (52°N, 107°W), 1980–1996

The daily mean winds observed by the Saskatoon MF radar are used to investigate the quasi 16-day oscillations in the mesosphere and lower thermosphere. Based on the wind data (every 3 km from 58 to 105 km, hourly from 1980 to 1996), the wave amplitudes and phases of all periods (2–24 days) as a function of height and time of year are available. Among them the 16-day wave occurs there mostly in winter with a maximum at ∼60–65 km and covers a large range of altitudes (up to 100 km); in summer, however, the 16-day wave is much weaker and confined to ∼85 km and above. The vertical wavelength in winter tends to be very long, but in summer it is a little shorter. After further investigation we found that the 16-day waves are extremely sensitive to the background mean winds; this explains their penetration, preferably in the westerly flow, from the lower atmosphere to the mesosphere in winter, or through a presumed ducting from the Southern Hemisphere to the Northern Hemisphere in Saskatoon summer. The year-by-year variations of the 16-day wave in this paper show little correlation with the winter stratospheric warming events but imply certain associations with the equatorial quasi-biennial oscillation both in winter and in summer. The global scale wave model has been run to get the annual (12 months) 16-day wave simulations from the ground to 200 km. The comparison with the observations at Saskatoon indicates good agreement in winter months but significant differences in summer.

Extra long period (20–40 day) oscillations in the mesospheric and lower thermospheric winds: observations in Canada, Europe and Japan, and considerations of possible solar influences

Abstract An extra long period (20–40 day) oscillation has been identified in the mesospheric and lower thermospheric (60–100 km) winds observed simultaneously by radars (MF, LF) at four sites from 70°N to 30°N in the northern hemisphere during the winter of 1995/1996. A long-term (1980–1999) investigation of this oscillation at Saskatoon and Collm is also carried out to obtain climatological and statistical characteristics. Spectral analysis has shown that this oscillation is a common feature of the winter (November–March) atmosphere, having strong amplitudes throughout the mesosphere (∼10 m/s ) and lower thermosphere (∼5 m/s ) , and being much stronger at mid-low latitudes. Although the oscillation has a climatology similar to the long period normal mode planetary waves (10–16 day), the phases at the various sites are very similar, and not consistent with a freely propagating wave. Comparisons with geomagnetic/solar wind parameters and solar UV radiation suggest that the oscillation could be related to the short-term solar rotation period (ca. 27 days) in some way. However the range of observed wind periods is very broad and this raises questions about this interpretation. Nevertheless the inter-annual variations of this 20–40 day oscillation indicate a weak 11-year solar cycle correlation in the mesosphere (positive) and the lower thermosphere (negative). Also, the cross-correlation between the winds and solar radiation shows significant quasi 27-day correlation and the wind lags behind the solar radiation a few days in the mesosphere. In general it is implied that the atmosphere could react to the solar activity in an indirect way due to certain dynamical mechanisms.

Gravity wave activity and dynamical effects in the middle atmosphere (60-90km): Observations from an MF/MLT radar network, and results from the Canadian middle atmosphere model (CMAM)

Abstract It has become increasingly clear that Gravity Waves (GW) have an essential and often dominant role in the dynamics of the Middle Atmosphere. This leads to them having strong impacts upon the thermal structure and the distribution of atmospheric constituents. However, the radar observations of GW have been limited in their latitudinal extent during the past decade, and although satellite observations are now significantly contributing, global-seasonal climatologies of important characteristics are still inadequate. With regard to models, the inclusion of GW-drag effects has been problematic. Usually no seasonal or latitudinal variation in the subgrid-scale GW-drag parameterization scheme is included, and varieties of parameterization schemes have been used. Although these often make conflicting assumptions, they generally produce similarly acceptable end-products, e.g. zonal-mean zonal wind fields. In this paper, we report upon the beginnings of a substantial program, using observations from a network of MF radars (North America, Pacific and Europe), and data from the Canadian Middle Atmosphere Model (CMAM). This model allows the tidal and planetary wave fields to be assessed, characteristics and climatologies of which are well known from the MF Radars. Here we focus upon the tides. There are useful similarities in the observed and modeled background wind and wave fields, and strong indications that the two non-orographic GW-drag parameterization schemes (Hines; Medvedev–Klaassen) have significant and differing effects upon the dynamics of the modeled atmosphere. It is shown that this comparison process is valuable in the evaluation, and potentially the optimization, of parameterization schemes.

Correlations of gravity waves and tides in the mesosphere over Saskatoon

Abstract Measurements in the Mesosphere and Lower Thermosphere (MLT), from the Saskatoon MF (medium frequency) radar (52°N, 107°W), are used for the years 1992 and 1993 to study gravity waves (GW) and their variability. GW-band time series (10–100 min; 2–6 h), their spectra and their 12, 24, 48 h oscillations are used. Fifty days of data from three seasons are selected, which are themselves marked by strong differences in middle atmospheric winds and solar tides. Encouragingly consistent modulations of the GW variances at 12, 24, 48 h periods are noted during the summer months, for both short events (a few days) and for the entire 50 days (for the 2–6 h). The inferred GW propagation directions (toward the NE) are internally consistent with calculations of GW propagation-directions using a new correlation method and consistent with independent measurements of gravity waves discussed in earlier articles from Saskatoon. In this season, all three oscillations are comparable and moderately large in the hourly-mean winds. During the winter, when the 12 h tide is dominant, there is apparent modulation of the GW variances for short events only, despite the size of the tidal wind oscillations (larger than summer). The inferred GW propagation directions are closest to eastward, although the inference is not strong, due to the weakness and variability of the modulations. Surprisingly, in the autumn months, when the 12 h tide is at its annual maximum, the modulation is very weak. However, an event of a few days was identified when some consistency was identified. Spectra from a Lomb–Scargle spectral analysis of the variances for nine years are also used to provide a climatology. There is dominance of peaks near 24, 12 and even 48 h during summer months with a much weaker tendency for peaks to occur in winter and autumn. The existence of peaks at 6 h, although possible when the propagation directions of the GW fluxes are isotropic, is actually a rare event. This also confirms earlier results from Saskatoon, that indicate anisotropy of the GW directions is usually quite strong. The intermittent nature of the GW modulation at tidal periods suggests strongly that the wave sources are intermittent in strength and direction and that the background wind at lower heights also contributes variability.

Observations of 7-d planetary waves with MLT radars and the UARS-HRDI instrument

Abstract Long period variations in the mesosphere wind have been observed for some time by ground-based radars. These planetary scale disturbances have reoccurring periods at or near 5–7, 10, and 16 days and at times dominate the wind field at mesospheric heights. Recently, due to the continuous operation of several of the MLT radars and the availability of measurements from the UARS satellite, it has been possible to compare observations during periods of large planetary wave activity. Wind measurements from four MLT radars; the meteor radars at Durham, NH (43°N,71°W) and Sheffield, UK (53°N,2°W) and MF radars at Urbana, IL (40°N,88°W) and Saskatoon, Canada (52°N,107°W) were compared with the HRDI measurements during intervals when 7-d planetary waves were present. Wind data from the HRDI instrument on UARS has been processed to show the latitudinal structure and the seasonal variation of the planetary scale wind variation. The phases and amplitudes of the waves as determined by both the satellite and the radars are in good agreement. The ground-based measurements show large modulation of tides by these long period components, and also show comparable responses of these low frequency components over thousands of kilometers. The satellite and the ground-based results both indicate a preponderance of wave occurrence during the equinoxes and at preferred latitudes.

Gravity wave spectra, directions and wave interactions: Global MLT-MFR network

Observations of winds and gravity waves (GW) by MF radars from the Arctic to the Equator are used to provide frequency spectra and spectral variances of horizontal motions, and information on the predominant azimuthal directions of propagation for the waves. The years used are mainly 1993/4; the height layer 76–88 km; and the GW bands 10 100 min. and 1–6 hrs. The high/mid-latitude locations of Tromsø, Saskatoon, London/Urbana, Yamagawa, generally demonstrate similar behaviour: the monthly spectra have slopes near −5/3 in winter months, but smaller (absolute) slopes at higher frequencies (<2 hrs.) in summer. Corresponding to this, the spectral densities (10–100 min.) are larger for conditions of higher mean background windspeed—this is related by means of a new correlation-vector technique to GW propagating anti-parallel to the mean zonal winds, and the closure of the solstitial mesospheric jets. Also consistent with this, the sizes and orientations of perturbation ovals (fitted to the wind variations), demonstrate strong semi-annual-oscillations (SAO), and generally similar monthly and latitudinal directions. This suggests strong control, especially of the high-frequency GW band, by the dominant zonal wind-structures of the mesosphere. In contrast the low-latitude locations of Hawaii and Christmas Island demonstrate uniquely different behaviours, with indications of significant inter-annual variability. The frequency spectra for all months tend to have smaller slopes at higher frequencies. Also the dependence of spectral density in both GW bands, upon background wind speed, is negative rather than positive, and is shown to be generally consistent with GW propagating parallel to the mean-global winds. This is consistent with weaker vertical shears in the zonal winds (76–88 km), and lower GW momentum depositions. The perturbation ovals reveal much weaker SAO, and more variable orientations, consistent with more dependency upon GW sources, and less control by the mean winds of the mesosphere.